Whilst this topic doesn't personally concern me in either direction (I usually like to play these games solo), I see no real reason to restrict the party size to 4, other than these possible problems.
1. Cinematic Dialogues with party interjections becoming strained the more party members you have. 2. Encounters designed for one party size not scaling well in comparison to another party size. 3. Combat "feeling too tedious" when you have to handle lots of characters. 4. They want to enforce a particular game experience, which they can do by limiting the party size to 4. Lowering the maximum party size makes decisions between which classes you want to take more stringent, which might be something they want to achieve.
Of these 4, I feel the hardest to address is the 1st one. I would guess that if this is something they are unwilling to budge on, it is the reason they are most likely against it. If there is a solution that deals well with this, then I could see a larger party size being allowed.
The 2nd 1 can be partially addressed by dividing XP between party members so the more party members you have, the less XP you get. This does not entirely solve the problem however, because in these types of game character's power does not scale linearly with level and so a party of 3 who is 1 level higher than a party of 6 is probably a bit stronger, especially at break points like 4 and 5. So if you were to balance party size this way, you would probably have to balance the experience around the maximum allowable party size, where reducing the number of party members would then also lead to a reduced difficulty for the player.
The third point is an entirely self inflicted problem by Larian, because they want us to watch fancy animations. If you add an option to skip combat animations, which pretty much every turn based strategy game has, then combat would actually be much, much faster.
The 4th point, you can implement as a soft enforcement. Have maximum allowable party size be a toggle in the options menu and set the default setting for it, to be whatever value provides their intended experience. For those that want to get their own experience, they can then manually override it. I also see no particular reason for the maximum to be 6 here, why not let people go up to 8 or even higher if they want to override this?
I guess point nbr 3 would depend on how Larian handle combat balance. If they could balance it without just adding more enemies then I think that combat could feel more interactive and have a better flow to it with 6 party members instead of 4.